Charge storage effects in silicon dioxide films

The dc properties of the system Si:SiO
2
(6000 Å), metal have been investigated at 200-400°C. The system shows unidirectional conduction at 400°C. Little or no current is observed when the silicon electrode is positive. There is a region of reversible charge storage and discharge at negative silicon voltages; the center of this region is around -2 volts when the contact metal is gold, but more positive when it is aluminum. The quantities of charge stored range from 4 × 10
¹²
to 2 × 10
¹⁴
charges/cm
²
. At still more negative silicon voltages there is a region of ohmic conduction, the resistivity at 400°C ranging from 10
¹²
to 10
¹⁴
Ω-cm. The charge storage, which is far greater than that anticipated from the oxide capacitance, can be shown to occur principally in the oxide under the metal contact, and not over a broad adjacent area. Storage of charge in the oxide causes an
n
-type shift of the underlying silicon surface. Charges can develop in the oxide as a result of nonelectrical processes, such as high-temperature gas baking treatments. Conversion of the upper 600 Å of the oxide to a phosphate glass eliminates both charge storage and ohmic conductivity in the oxide. The results described are interpreted in terms of an electrochemical potential existing across the oxide, and the presence of mobile charged species within the oxide. Charge storage can occur as the result of double-layer formation by the mobile charged species, or of its electrochemical discharge at the electrodes. The unidirectionality of the ohmic conduction is considered to result from electrolytic rectification at the silicon-oxide boundary.